Measuring based on an oscillating sensor of angular velocity has proved to have a simple principle and to provide a reliable way of measuring angular velocity. A certain known primary motion is being generated and maintained in the oscillating sensor of angular velocity. The movement, which the sensor is designed to measure, is detected as a deviation of the primary motion.
An external angular velocity affecting the sensor in a direction perpendicular to the direction of motion of the resonators induces a Coriolis force on the seismic mass perpendicular to its direction of motion. The Coriolis force being proportional to the angular velocity is detected from the oscillation of the mass in, for instance, a capacitive manner.
The biggest problem concerning micro-mechanical oscillating sensors of angular velocity is the so called quadrature signal, which is caused by poor dimensional accuracy of the structures. Resonators manufactured in the methods of micro-mechanics may have faulty tolerance in the orthogonality of their directions of motion, which gives rise to a signal, called a quadrature signal, which, at worst, is even hundreds of times the strength of the full scale indication of the angular velocity signal.
The angular velocity signal to be measured, the signal being proportional to the speed of the mass, is luckily at a 90 degree phase shift in relation to the quadrature signal, and thus the quadrature signal will disappear in an ideal demodulation. Since it is considerably much larger than the signal to be measured, it limits, however, the dynamics of the signal. In addition, the biggest disadvantage of the quadrature signal is the fact that it, due to phase shifts of the electronic signals, unless compensated, impairs the stability of the null point of the sensor.
Sensors of angular velocity according to prior art have also been designed, wherein attempts have been made to compensate the quadrature signal. One such quadrature signal compensation solution for a sensor of angular velocity according to prior art is the, so called, feed-forward compensation, wherein a force modulated by the detected primary motion is being fed into the detecting resonator in opposite phase in relation to the quadrature signal. This way of compensation is not very useful, since it merely shifts the stringent phase stability requirements of the electronics from the demodulation to the compensation block.
The oscillating structure can also be bent by static forces, and then the phase stability requirement of the electronics is essentially reduced. Among others, U.S. Pat. No. 6,370,937 describes such a solution for a sensor of angular velocity according to prior art. In the solution for a sensor of angular velocity described in the U.S. patent, the inclination of an electrostatic torsion resonator can be adjusted by means of an electrostatic force.
Another method according to prior art, more feasible than the previous one, for compensating the quadrature signal of a sensor of angular velocity, consists of generating, by means of a static quantity, a force modulated by the motion, which force compensates the quadrature signal caused by a residue in the spring force. A compensation method like this is superior to statically twisting the structure, since it allows mechanical structures of considerably higher stiffness. As the compensating force in such a solution always is in phase with the motion, it puts no additional demands on the phase control of the electronics.
U.S. Pat. No. 5,866,816 describes a solution for a sensor of angular velocity according to prior art. In the solution for a sensor of angular velocity described in the U.S. patent, the quadrature signal of a piezoelectric bar resonator can be compensated by means of piezoelectric forces by using a static bias voltage. Also an electrostatic force can work in the described manner, provided that electric field, asymmetric in the direction of the motion, can be provided.
Electrostatic compensation of the quadrature signal can be considered prior art in connection with linearly oscillating comb resonators. U.S. Pat. No. 5,992,233 describes a prior art solution for a sensor of angular velocity. In the solution for a sensor of angular velocity described in the U.S. patent, the electrode combs, parallel to the direction of the motion, are biased in such a way, that a motion of the resonator sideways in relation to the comb structure (direction Y in the U.S. patent) changes the area of the capacitor plates of the comb structure, which generates a linear, amplitude dependent force in the orthogonal direction (direction X in the U.S. patent).
One of the greatest advantages of the electrostatic quadrature compensation is that, by means of electronics, it can be made adaptive. U.S. Pat. No. 5,672,949 describes a solution for a sensor of angular velocity according to prior art. In the U.S. patent, the changes in the quadrature signal caused by mechanical twisting, aging, or various temperature dependencies continuously can be compensated based on the sensor's detection signal.
The structures according to prior art described above are not, however, suitable for application in sensors of angular velocity, wherein the primary motion is one of rotary oscillation. This kind of structure is particularly suitable in solutions requiring good resistance to vibration and impact.
An object of the invention is, in fact, the provision of a structure of an oscillating sensor of angular velocity, wherein electrostatic compensation of the quadrature signal is implemented such that it is particularly well suited for micro-mechanical rotary oscillating sensors of angular velocity, in comparison with prior art solutions.